|
|
| ||||||||||||||||||||
| |||||||||||||||||||||
A spotlight is about to be put on aircraft wiring and electrical systems. Operators can expect new inspection and reporting requirements. Indeed, the contents of a draft questionnaire to the industry suggests a major effort to better understand the condition of wiring and wiring-related maintenance practices in the fleet.
Although Federal Aviation Administration (FAA) officials claim the timing is only coincidental with the Sept. 2 crash of Swissair Flight 111, a program of increased inspections of aircraft wiring is expected to be launched within the next 30 days.
A massive electrical failure is the scenario now under closest scrutiny by investigators of the Swissair crash. The aromatic polyimide (Kapton) wiring on the aircraft, although resistant to burning at lower temperatures, flares brilliantly under the intense 10,000 degrees F heat of an electrical short circuit. Indeed, Kapton insulation carbonizes under high temperature, and the charred insulation can itself become a conductor.
One Man's Experience With Polyimide Wire Insulation
"One day I demonstrated for a Boeing engineer what new Kapton wire will do when it is flexed. I took a short piece, flexed it back and forth, several times, until the insulation material shattered. We were looking at bare wire! Imagine what happens in an aircraft when, in routing a wire bundle, it is stressed around a sharp bend, next to other wires or structure. Given the normal airplane vibration, you have a potential arc tracking condition.
At the extreme temperatures of an electrical fire, Kapton changes into carbon and becomes a conductor...You do not have a lot of time to isolate which bus is causing the smoke/fire. The fire will jump to other wire bundles and it's like strings of firecrackers being set off."
Patrick Price (retired Boeing wire technician)
Use Of Polyimide Wire More Restricted Today
For this reason, the industry has looked to other types of wire insulation.
Kapton's use is more limited on aircraft currently in production. For example, according to Airbus Industrie's Mark Luginbill, Kapton wire is restricted to "benign environmental applications, such as the cabin."
The dominant form of wire on Airbus aircraft features an insulation construction known as "KT." Kapton, next to the conducter, is protected by an outer layer of Teflon (i.e., polyimide with a topcoat of PTFE, polytetrafluorethylene). According to various wire experts, PTFE-coated wire bends easier and is arc-temperature resistant. Unlike Kapton, it is not fireproof in the face of chemical (as opposed to electrical) fires. Moreover, according to a 1994 airworthiness directive concerning wire chafing on Boeing 747s, "polyimide insulated wire (Kapton) could possibly be superior to softer types of wire insulation, such as PTFE insulated types of wire."
A wire known generically as TKT (for Teflon-Kapton-Teflon) has been installed on Boeing 757s and 737s built since 1992. According to Luginbill, Airbus engineers are evaluating TKT. "There's nothing yet to say it's better than PTFE."
"We feel that PTFE is safe, and there is no one wire type that fits all applications," he maintained.
Dr. Armin Bruning, Ph.D., whose expertise is in aircraft wiring, agrees. "The routing, the impedance of the electrical systems, and other factors dictate the type of wire to be used," he explained.
Wiring 101
A Very Brief History of Aircraft Wire Insulation
* During the 1970s and 1980s the most frequently used airframe wire constructions were based on aromatic polyimide (i.e., Kapton). However, during the 1980s, its arc track performance became a major industry issue.
* Cross-linked ethylene tetrafluoroethylene (XL-ETFE), the second most widely used aerospace insulation system, was not considered an acceptable alternative for use in the airframe by some OEMs (original equipment manufacturers) and end users because of its smoke and fire properties.
* Years of development and testing resulted in a new family of hybrids that combined the preferred properties of polytetrafluoroethylene (PTFE) and polyimide.
Source: Pat Cahill, Fire Safety Section, FAA William J. Hughes Technical Center
New program recommended nearly two years ago
The FAA's soon-to-be-announced program is an outgrowth of the White House Commission on Aviation Safety and Security (a.k.a., the "Gore Commission"). The commission's February 1997 final report recommended expanding the structural inspections of aging airliners to include electrical and other internal systems.
After months of behind-the-scenes work by government and industry experts, that effort is about to be unveiled. From records of these meetings and other internal documents, these aspects of the program are evident:
* Older jets from as many airlines as possible. The focus initially may be on soon-to-be-retired aircraft (those over 20 years old). However, it is possible that aircraft with as few as 30,000 hours may come under the inspection regime. The Swissair MD-11 that crashed had accumulated some 35,000 hours, and recent wiring inspections of 737s included all aircraft with more than 30,000 hours (see ASW, May 25).
* Zone-by-zone scrutiny. Wire inspections will include: areas normally hidden from view, areas in close proximity to flammable liquids and gases (fuel vapors, oxygen, etc.), high electric current draw areas, high vibration areas, corrosion-prone areas, high maintenance traffic areas, etc.
* Detailed reporting. For ease of data compilation, a standardized reporting form of findings and actions taken is likely to be required. The 100-page service bulletin Boeing issued Sept. 25, 1997, concerning the electrical components of 747 center wing fuel tanks, is seen as the model for both instructions for conducting inspections and for reporting the results.
* A questionnaire. Operators can expect at least a five-page questionnaire calling for essay-type explanations of any systemic wiring problems, the burden of unscheduled maintenance actions due to wire corrosion, chafing or insulation breakdown, the criteria used to replace cracked wire, and other items. >> Luginbill, 703/834-3596; Bruning, 703/481-1233; Price, 253/529-2718 <<
Questions Operators Can Expect About Wiring
* How many unscheduled maintenance actions have been written for the removal of wiring bundles, harnesses, connectors or relays because of corrosion, chafing or insulation breakdown at this and the last heavy maintenance visit?
* Is all the airplane wire inspected? If not, why not?
* Have any types of wire been a problem in the past? Why?
* Do dedicated wiring specialists inspect and do wire repairs or do A&P mechanics do the repairs/inspections?
* What inspection and maintenance tasks are done on circuit breakers and panels?
* How do you insure that the wire inside a bundle has not deteriorated to the point where there is a serious safety concern?
Source: FAA Aging Transport Airplane Systems Checklist, Revision 3
from this link
